Process for the production of electricity by means of a fuel cell



Patented June 19, 1934 UNITED STATES PROCE S S FOR TRICITY BY Thepresent application is a continuation in part of application Serial No.440,164 by Herbert H. Greger, filed March 29, 1930.

This invention relates to the production of electricity by directcombustion of a gaseous fuel in what is known as a gas cel It is wellknown that an electromotive force is produced when oxygen and acombustible gas are in contact with suitable electrodes and a suitableelectrolyte in such a manner that the gases cannot mix. If theelectrodes are connected by a conductor an electric current will flowthrough the cell and the outer circuit.

The source of this current is the chemical energy, which is liberated bythe oxidation of the combustible gas or of the fuel in general. Theoxygen which is required forthis combustion is transported by theelectrolyte from the oxygen electrode to the gas electrode. It isobvious that only such electrolytes will be suitable for this transportof oxygen, which contain oxygen as a constituent, that is to say, whicharesuitable to produce anions containing oxygen, such as for instancesodium sulphate or sodium carbonate. Any kind of fuel gas may be used,such as hydrogen, carbonmonoxide, methane, coal gas,.nat ural gas, watergas, etc.

The anion causes the oxidation of the fuel and liberates negativeelectrons to the gas electrode, imparting to this a negative charge. Atthe same time the cation liberates its positive charge on the oxygenelectrode and is oxidized by the oxygen present at this electrode, thusleaving the composition of the electrolyte unchanged.

(1) ZN8++ LO2=NEL2O+2+ (on the oxygen electrode) I (2) (a)CO3"+CO=2CO2+2" (b) CO3 +H2=CO2+H20+2- (on the gas electrode)Na2O+CO2=Na2CO3 (regeneration of NazCOs) It may be assumed that sodiumcarbonate is used as an electrolyte, forming Na+-ions and Cor -ions. isof no importance in these considerations.

If CO or Hz is passed to the gas electrode, this gas will have areducing action on the CO3-ion and a very small amount of this anion maybe reduced. At the same time its electric charge will be liberated tothe said gas electrode, imparting to the latter a negative charge; Thecharging of the electrode will cease after an equilibrium between theelectrostatic energy on the electrode and the chemical energy of the,librium between the electrical and chemical en- The composition of theelectrodes I operating temperature of the cell.

THE PRODUCTION OF ELEC- MEAN S OF A FUEL CELL Herbert Hans Greg'er,Akita, Japan is Claims. (01. 136-86) toastegaseous fuel has beenobtained. The corresponding process will take place on the oxygenelectrode, the cation will be converted to. sodium oxide and itspositive charge liberated until equi- '60 ergy is obtained. Ondischarging the electrodes by connecting them through a conductor, theprocesses on both electrodes will proceed as long as gas and oxygen .areavailable. In oxidizing 7 one gram-molecule of CO two faradays ofelectricity (96540 ampere seconds) are produced, while the voltageobtained depends on the free energy liberated during oxidation. Thisfree energy varies somewhat with the temperature at which the process isoperated;

In order to explain the process fully we must assume that on the gaselectrode carbon dioxide 18 formed from the electrolyte, while on theoxygen electrode an equivalent amount of sodium oxide is produced. Thesodium oxide is transferred to the gas electrode and there absorbs orreacts with the carbon dioxide, regenerating sodium carbonate. Alsosmall amounts of sodium hydrate or hydroxide may be formed if steam ispresent on the gas electrode or when hydrogen is chiefly used as a fuel.

Various attempts have been made in the past to construct fuel cells forpractical purposes. These attempts may be divided into two classes,those using aqueous solutions as an electrolyte and operating at roomtemperature (20 C.) While the other method use molten salts as anelectrolyte and a temperature above the melting temperature of thelatter. The fuel cell which I have invented belongs to the second class.In thisclass the most important conditions for operating andconstructing a fuelcell, which requires careful attention, comprise:

(a) The fuel gas and the oxygen must react readily on the electrodes inorder to become electromotively active. The tendency of these gases toreact, which is very slight at ordinary temperature, increases howeverwhen the temperature of operation of the cell is increased.

(2)) The composition of the electrolyte must not be changed throughchemical reaction with the fuel gas, its products of combustion andoxygen. Also it must not be changed by physical factors, such asevaporation.

(c) The electrode material for the cell must neither be attacked bu thefuel gas, its products of combustion and oxygen, nor by the electrolyte.

(d) The construction material for the cell must not be attacked by theelectrolyte at the (e) Absolute gas tightness is required in order toavoid any mixing of the fuel gas and oxygen.

Unfortunately the number of electrolytes which may be used withadvantage at increased temperatures is very limited, especially by therequirements of item b. In previous processes alkali metal carbonateswere used as an electrolyte, which gave satisfactory results with regardto the chemical stability of this compound. However the meltingtemperature of sodium and potassium carbonate is exceptionally high andeven their mixture of 50 molar per cent of each substance melts at 704C. A cell'in which this electrolyte would be used would have a workingtemperature of nearly 800 C., a temperature which is too high to makethe construction and operation of a large scale cell commerciallyfeasible. The chief disadvantages involved in too high a temperatureare:

(a) Metals which must be used in various places of the cell aselectrodes, terminals, casings, pillars, etc., are subject to a highcorroding action.

(b) The electrolyte has an excessive vapor pressure and its evaporationmay have various undesired consequences, for instance the cell may dryout. n (c) The refractory material which must be used in any case in theconstruction of a fuel cell is subject to excessive corrosion by theelectrolyte, except'in the case where very pure magnesia bricks areused, which however are very expensive.

(d) The uniform and economical heating of the cell would provide manydifiiculties' I have invented a new process for the production ofelectricity in gas cells, in which the above mentioned difficultiesconsequent upon too high a temperature are satisfactorily overcome. Inthis process an electrolyte of relatively low melting temperature isapplied, whereby entirely new conditions for the construction andoperation of fuel cells have been produced.

The new electrolyte consists of carbonates of the alkali and alkalineearth metals to which halide salts of the alkali and alkaline earthmetals are added as a neutral solvent, which among other solvents gavethe greatest satisfaction. It will be understood that numerous mixturesand combinations of these chemicals may be made.

Several examples of mixtures which are suitable for industrialapplication will give a clear idea of the usefulness of the newelectrolyte:

In percent by weight Melting point 475 In the above table the mixtures Iand III, further III and IV, as well as VII and VIII are each identicalbecause after melting they form mixtures that have the same meltingpoint.

According to the law regulating the action of substances and of theexchangeability of ions in molten mixtures of salts it is possible tapoduce a certain definite salt mixture from vaiious raw materials,provided that the quantities of the single ions that are introduced intothe mixture are always the same. For instance, it may seem to bedesirable by economic or other considerations to replace the amount ofKCl in II by a corresponding amount of BaClz. This necessitates thereduction inthe amount of BaCOs by an equivalent amount of barium-ion,while the equivalent amount of COa-ion is introduced aspotassium-carbonate.

By means of these electrolyte mixtures it became possible to operatefuel cells at temperatures between the melting temperature of theelectrolytes and about 700 C., which from practical reasons may beconsidered to be the upper limit at which industrial scale cells couldbe operated. The ordinary operating temperature for the process may beconsidered to be at about 550 to 650 C.

The new conditions for the construction and operation of the new fuelcell may be summarized as follows:

At the melting temperature of the electrolyte an evaporation of thispractically does not exist 100 and at the working temperature of thecell at about 550 to 650 C., the evaporation is negligible. Cheapmetals, such as iron or calorized iron may be used with advantage as aconstruction material for the cell, its heating arrangements and fordevices for heat exchange between incoming and outgoing gases. The highdegree of heat economy which thus becomes possible without anyappreciable dificulty, will contribute greatly to the efiiciency of theprocess. Also the more no common kinds of refractory material may beused without danger of corrosion.

The new electrolyte had to undergo very thorough tests, because it couldbe considered as satisfactory only if those substances which are admixedto the carbonates as a solvent, would not produce any chemicalpolarization in the cell and proved to be entirely neutral. That theadmixed halide salts of the alkali and alkaline earth metals are reallyneutral may be seen in the fact that hydrogen and carbon monoxide gavealmost the theoretical electromotive forces in a cell in whichnumerous'diiferent mixtures of the new electrolyte were tested togetherwith oxygenelectrodes of nickel, nickel-chromium-steel and magnetite andgas electrodes of nickel, iron and carbon.

Electromotive force at 600 0. I 332;? $32325 Hydrogen l. 14 l. 16 Carbonmonoxide l. 04 1. 06

The theoretical electromotive forces and those experimentally producedare sufliciently in agreement to prove that no other reaction than theoxidation of the fuel gas takes place. From this reason the electrolytemust be considered as specially suitable for being used in fuel cells.

Whereas I have described my invention by reference to specific formsthereof, it will be understood that many changes and modifications maybe made without departing from my invention. Y

I claim:

1. The process of generating electricity which comprises introducing afuel gas into a gas cell at the gas electrode thereof where it contactsan electrolyte comprising a fused mixture of alkali metal and alkalineearth metal carbonates and halides, and reacts chemically therewithreleasing electrical charges which are imparted to said gas electrodeand simultaneously introducing an oxygen-containing gas at the oxygenelectrode where it contacts said electrolyte and reacts chemicallytherewith, releasing electrical charges which are impartedto said oxygenelectrode, the said cell being operated at a temperature between themelting point of the electrolyte and 700 C.

2. The process of generating electricity which comprises oxidizing afuel gas at the gas electrode of a gas cell by means of an electrolytecomprising a fused mixture of carbonates and halides of the alkali andalkaline earth metals, the melting point of the said mixture of saltsbeing below 700 C., and simultaneously reducing oxygen gas at the oxygenelectrode of the said gas cell.

, 3. The process of generating electricity which comprises oxidizing afuel gas at the gas electrode of a gas cell by means of a carbonatecontained in an electrolyte comprising a fused mixture of sodium,carbonate, potassium carbonate, barium carbonate, sodium chloride,potassium chloride, barium chloride, sodium fluoride, and potassiumfluoride, the melting point of the fused mixture being below 700 C., andsimultaneously reducing oxygen gas by reaction with positively chargedions at the oxygen electrode of the said gas cell.

4. The process of generating electricity by oxidizing carbon monoxideand hydrogen at the gas electrode of a gas cell by means of a carbonatecontained in a fused mixture of sodium carbonate, potassium carbonate,barium carbonate, sodium chloride, potassium chloride, barium chloride,sodium fluoride and potassium fluoride the melting point of the fusedmixture being below 700 C., and simultaneously reducing oxygen byreaction with positively charged ions at the oxygen electrode of thesaid gas cell.

5. The process of generating electricity which comprises introducing afuel gas into a cell at the gas electrode thereof where it contacts anelectrolyte comprising a fused mixture of alkali metal and alkalineearth metal carbonates and halides having a melting point below 0., andreacts chemically therewith, releasing electrical charges which areimparted to said gas electrode and simultaneously introducing an oxygencontaining gas at the, oxygen electrode where it contacts the aforesaidelectrolyte and reacts chemically therewith, releasing electricalcharges which are imparted to said oxygen electrode.

6. The process as set forth in claim 5 in which the fuel gas is carbonmonoxide and hydrogen.

'l. The process as set forth in claim 5 inwhich the electrolytecomprises a sodium carbonate, potassium carbonate, barium carbonate,sodium chloride, potassium chloride, barium chloride, sodium fluorideand potassium fluoride.

8. The process of generating electricity which comprises introducing afuel gas into a cell at the gas electrode thereof, where it contacts anelectrolyte comprising a fused mixture of alkali metal and alkalineearth metal carbonates and halides having a melting point below 600 0.,and reacts chemically therewith, releasing electrical charges which areimparted to said gas electrode and simultaneously introducing an oxygencontaining gas at the oxygen electrode where it contacts the aforesaidelectrolyte and reacts chemically therewith, releasing electricalcharges which are imparted to said oxygen electrode.

9. The process of comprises introducing a fuel gas into a cell at thegas electrode thereof, where it contacts an electrolyte comprising afused mixture of alkali metal and alkaline earth metal carbonates andhalides having a melting point below 500 C., and reacts chemicallytherewith, releasing electrical charges which are imparted to said gaselectrode and simultaneously introducing an oxygen containing gas at the.oxygen electrode where it contacts the aforesaid electrolyte and reactschemically therewith, releasing electrical charges which are imparted tosaid oxygen electrode.

10. In a fuel cell of the class described, an electrolyte comprising afused mixture of alkali and alkaline earth metal carbonates and halides,the said mixture having a melting point below 700 C.

11. In a fuel cell of the class described, an electrolyte comprising afused mixture of sodium carbonate, potassium carbonate, bariumcarbonate, sodium chloride, potassium chloride, barium chloride, sodiumchloride and potassium fluoride, the melting point of the fused mixturebeing below 700 C.-

12. In a fuel cell of the class described, an electrolyte comprising afused mixture of alkali metal and alkaline earth metal carbonates andhalides, the said mixture having a melting point below 600 C.

, 13. In a fuel cell of the class described, an electrolyte comprising afused mixture of alkali metal and alkaline earth metal carbonates andhalides, the said mixture havin a melting point below 500 C.

' T H. GREGER.

generating electricity which fused mixture of

